An X-ray tube device having an anode rotation mechanism for increasing an allowable load by transferring an electron collision cross section is very frequently used in the field of X-ray imaging apparatuses including an X-ray inspection apparatus and an X-ray image diagnostic apparatus such as an X-ray CT apparatus.
As shown in figures in the document “Johns, H. E., et al: The Physics of Radiology.3rd. ed., Charles C Thomas Publisher, Springfield, 1969” (The same drawing is in Ishiyaku Publishers, Inc.: Medical Radiation Science Course 13, “Radiation Diagnostic Instrument Engineering”, page 7, FIG. 1—1), an anode of the X-ray tube device includes a rotator and an umbrella-shaped target and is rotated in the same principle of induction motor. An area of the electron collision cross section of the target is extended by rotating the target, wherein in case of a short-time load, an input for a unit area of a focus can be greatly increased. Accordingly, an X-ray tube device having large capacity can be realized. The anode having a rotor coil is rotated in the X-ray tube device within a rotating magnetic field generated by supplying an electric current to a stator coil winded around a stator provided outside the X-ray tube.
As described above, the anode is rotated in the same principle as that of induction motor. A difference to induction motor is that a glass or a metal covering the X-ray tube exists between the stator and the rotator, and so the gap is large.
In an X-ray generating device using thus constructed rotary anode X-ray tube, a single-phase or three-phase AC voltage is supplied to the stator coil inside the anode rotation mechanism before radiating X-rays from the X-ray tube and a rotating magnetic field is generated, and thus the anode is rotated. After the rotation of the anode is accelerated and the rotation number becomes steady so that generated torque of the motor coincides with a load torque on the motor determined by the mechanical system of the anode rotation mechanism, a DC high voltage is applied between the anode and the cathode of the X-ray tube from an X-ray high voltage generating device, whereby X-rays are radiated and scanning is started.
When a portion for diagnosis of an object to be examined is scanned, in the X-ray tube, electronic beams are radiated from the cathode, and collided with and reflected by the anode target to generate X-rays. Because the electron beams generated from the cathode have enormous energy, the anode target is rotated as described above for the purpose of avoiding instantaneous burning of the anode target collided with the electronic beams.
Japanese Unexamined Patent Publication No. 2000-150193 discloses a mechanism of controlling rotation drive of the anode in three operation modes by supplying a voltage to the anode rotation mechanism.
The first operation mode is a starting mode, which requires large activating torque. Accordingly, a high AC voltage of, for example, about 500V is applied to the stator coil to activate the anode. The second mode is a steady mode, in which after the anode is activated, its rotation number reaches a predetermined number, i.e., it coincides with a torque determined by a system of the anode rotation mechanism. Because this driving torque is smaller than the starting torque, it is enough to supply a low AC voltage of about 200V to the stator coil. The third operation mode is a breaking mode to stop the anode rotation, in which a DC voltage of about 120V is supplied to the stator coil to put brake on the DC voltage. Here, the operation time of the starting mode is the time until the rotation number of the anode reaches a predetermined number. As disclosed in, for example, Japanese Unexamined Patent Publication No. Sho.53-78191, this time can be accurately measured by installing a rotation number meter to an anode rotation shaft and directly detecting the rotation number. However, it is technically difficult to install the rotation number meter under the circumstance of high temperature, vacuum, and high voltage and within a limited space. According to the conventional technique, a time until the anode rotation number reaches the predetermined number is previously measured, and the time, referred to as X-ray radiation waiting time hereinafter, is set to an X-ray high voltage generating device. Accordingly, in X-ray imaging, a rotation driving signal is output from the X-ray high voltage generating device to the anode driving mechanism and X-rays are radiated to start scanning after a lapse of the predetermined X-ray radiation waiting time. That is, X-rays are radiated when the anode rotation number reaches the predetermined number. In short, when an image is obtained by an X-ray imaging apparatus, an anode driving signal is output from the X-ray high voltage generating device to the anode driving mechanism, an X-ray radiation waiting time is preset so that the anode rotation number reaches the predetermined number by driving the anode to rotate with the anode rotation device, a DC high voltage is output from the X-ray high voltage generating device after a lapse of the X-ray radiation waiting time and applied to the X-ray tube, and thus X-rays are radiated from the X-ray tube.
However, the X-ray radiation waiting time (the time until the rotary anode reaches a predetermined rotation number) depends on the following conditions:
(1) Effects of Temperature of Stator Coil
A time until the anode reaches a predetermined rotation number, e.g., a steady rotation number of 8000 rpm is around five seconds when the stator coil is cold. However, it is around six seconds when the stator coil is warm after several times of imaging. That is, in the state where the stator coil is warm, the time until the anode reaches the predetermined rotation number is prolonged.
The reason is that a resistance of the stator coil increases to reduce a current. If the X-ray radiation waiting time until the anode reaches the predetermined rotation number is set assuming a condition that the stator coil is warm (e.g., six seconds in the state where the stator coil is warm), a wasted time (e.g., one second) to X-ray radiation appears in the state where the stator coil is cold. When an object is observed and an imaging position is determined with X-ray fluoroscopy as in, for example, gastric contrast examination using barium, the wasted time becomes a factor of losing scanning timing by just one second or disturbing improvement of throughput of the X-ray image diagnostic apparatus. Accordingly, it is preferable to reduce the wasted time to be as small as possible. Further, in a fluid volume inspection apparatus using an X-ray tube device, because a passing speed can be improved by shortening the X-ray radiation waiting time, inspection time can be shortened.
(2) Effects of Fluctuation of Power Supply Voltage of Anode Driving Mechanism
An anode driving mechanism which rotates the anode by applying a single-phase or three-phase AC voltage to the stator coil and generating a rotating magnetic field usually includes an inverter circuit for converting a commercial AC power supply voltage into DC voltage, and further converting this DC voltage into a single-phase or three-phase AC voltage. An output voltage from the inverter circuit fluctuates in response to the commercial AC power supply voltage. Because a torque generated in the anode driving mechanism is approximately in proportion to square of the voltage applied to the stator coil, when the commercial power supply voltage fluctuates, the torque generated in the anode driving mechanism greatly fluctuates. Accordingly, the time until the anode rotation number reaches the predetermined number also changes. However, no special measure has been taken for this phenomenon.
(3) Other
In addition to (1) and (2) listed above, consideration of following matter is also necessary because a rotational property of the anode changes due to the temperature of the anode and a change of the frictional force of the anode rotation shaft.
The time until the anode rotation number reaches a predetermined rotation number fluctuates due to various factors. Therefore, in a conventional method of setting the predetermined X-ray radiation waiting time, it is necessary in consideration to the conditions based on the above (1) to (3) to set a sufficient X-ray radiation waiting time by separately preparing an interlock mechanism or the like for constantly stopping an X-ray radiation signal for a wasted time of 0.5 to 1 second after activating the rotary anode, as described in Japanese Unexamined Patent Publication No. Hei.5-114497 and in Japanese Patent Publication No. 3276967.
Further, if X-ray radiation is started before the anode rotation number reaches a predetermined rotation number for reasons such that a scanning timing is lost, a time from scanning preparation to scanning is prolonged, or any circumstance occurs, there is concern that fever of the anode increases to induce discharge and thus shorten life duration of the X-ray tube.
Japanese Unexamined Patent Publication No. Hei.5-114497 and Japanese Patent Publication No. 3276967 disclose a structure in which electric power consumption is detected from a reactive power or a power factor and compared with a preset value of power consumption in the predetermined rotation, and X-ray radiation signal is shut off when slippage is larger than the rated value.
According to the above construction, because power consumption is detected in accordance with the relational expression “active power=power consumption+reactive power”, it is necessary to take into consideration a phase difference in calculating reactive power or power factor. Accordingly, the power detecting mechanism becomes complicated and so the cost for the detection device becomes high.
Further, the electric power supplied from the inverter type driving circuit used as an anode driving mechanism fluctuates in accordance with the commercial AC power supply voltage as described above and is approximately in proportion to the square of the voltage applied to the stator coil. Accordingly, when the commercial power supply voltage fluctuates, the voltage to be supplied greatly fluctuates particularly in activating the inverter type driving circuit, thereby values of voltage and current detected when the anode have low reliability, and cannot be used for detection of the anode rotation number at a time of starting operation. Therefore, in the conventional technique, the above interlock mechanism is necessary. Although the interlock mechanism can shut off the X-ray radiation signal after the anode starts to rotate, it cannot adjust the X-ray radiation waiting time until the anode rotation number reaches a predetermined number.
Further, according to the conventional technique, it is necessary to determine a power consumption preset value in accordance with individual difference, aging, and types of X-ray tube. It is necessary to determine the power consumption preset value by practically driving and measuring X-ray tubes one by one, which requires so much energy.